1. Field of the Invention
This invention relates to improvement in III group nitride system compound semiconductor light emitting element.
2. Description of the Related Art
Conventionally, in making the III group nitride system compound semiconductor light emitting element (hereinafter also referred to simply as light emitting element), III group nitride system compound semiconductor layer is grown on the mirror-finished surface of a sapphire substrate through buffer layer. Critical angle at the interface of sapphire substrate and III group nitride system compound semiconductor layer is no more than about 47 degree because there is a big difference in refractive index therebetween. Therefore, some component of light emitted from the III group nitride system compound semiconductor layer can be returned to the semiconductor layer while being subjected to total reflection at the interface. Light returned to the semiconductor layer is attenuated due to scattering or absorption in crystal of the semiconductor layer. Thus, the big difference in refractive index between sapphire substrate and III group nitride system compound semiconductor layer prevents light emitted from III group nitride system compound semiconductor layer from being taken out efficiently.
So, it is suggested to form a pattern on the surface of sapphire substrate (Japanese patent application laid-open No.2001-267242 (prior art 1), Kazuyuki Tadatomo et al., “High Output Power InGaN Ultraviolet Light-Emitting Diodes Fabricated on Patterned Substrates Using Metalorganic Vapor Phase Epitaxy”) For example, prior art 1 discloses to form a pattern of ridges and grooves (depth: 1.5 μm) of 3 μm wide respectively on sapphire substrate by photolithography. Thereby, light entering at a big angle into the interface of sapphire substrate and III group nitride system compound semiconductor layer can be taken out from the sidewall of groove to enhance the light extraction efficiency.
Also, Japanese patent application laid-open Nos. 10-312971 and 2001-349338 disclose partially relevant techniques to the invention.
On the other hand, by patterning the surface of substrate, the lateral growth of III group nitride system compound semiconductor layer based on the sidewall of groove can be promoted. This prevents penetrating crystal transition in the vertical direction of III group nitride system compound semiconductor layer from occurring. Therefore, the crystal quality can be enhanced.
However, from the research of the inventor, it is found that it is difficult to promote uniformly the lateral growth on the entire surface of wafer even if the entire surface of wafer is patterned. Namely, it is difficult to grow III group nitride system compound semiconductor layer with a good crystal quality on the entire surface of wafer. As a result, the patterning of substrate causes a reduction in yield and an increase in the manufacturing cost of light emitting element.
Further, when III group nitride system compound semiconductor layer is grown in the groove formed by patterning, a cavity may be generated in the groove (See prior art 1). In this case, there occurs a big difference in refractive index between the cavity and III group nitride system compound semiconductor layer. Therefore, light emitted from III group nitride system compound semiconductor layer will be reflected on the wall of cavity. As a result, the light extraction efficiency lowers.
In addition, sapphire, which is commonly used as transparent substrate material, is difficult to process since it is hard and fragile. In other words, it limits the degree of freedom in forming the uneven pattern on the surface.